1. Field of the Invention
The present invention relates to method and apparatus for the extraction and recovery of zinc from zinc-bearing materials (solutions or solids) through a process carried out in a sulphuric acid media by leaching the zinc followed by precipitation of zinc as oxide and upgrading zinc content in precipitates by flotation or granulometric sizing.
2. Description of the Related Art
Acid soluble zinc ores (zinc oxide, zinc carbonate, zinc silicate, etc.) have been used in the production of zinc metal since the very beginnings of the industry. The leaching of theses ores and purification of impurities associated with these ores are well established. Wood, in a paper published in the Journal of Metals, Vol. 29, page 7, incorporated herein by reference, describes a process for the hydrometallurgical treatment of zinc silicate. U.S. Pat. No. 3,954,937, incorporated herein by reference, provide a process which utilizes a series of leaching tanks in which the acidity is progressively increased over a period of at least three hours, in such a quantity that the final acidity of the solution will be 1.5 to 15 g/l, while carefully maintaining the temperature at 70xc2x0 to 90xc2x0 C., thereby inducing the lixiviation of the material and simultaneously the reprecipitation of silica in a crystalline form which can be eliminated by filtration. U.S. Pat. No. 3,656,941, incorporated herein by reference, describes a process for the recovery of metal values from a siliceous zinc ore whereby the ore is treated with sulphuric acid in a continuous manner in stirred tanks. U.S. Pat. No. 5,585,079, incorporated herein by reference, describes a method for recovering zinc from a zinc oxide bearing material containing silicates. According to this method, the leaching of zinc compounds is carried out in circumstances where the silicates remain undissolved and consequently do not cause filtering problems. The silicate bearing residue is treated in a Waelz process.
Although, there are many processes that can provide a method to dissolve zinc from the acid soluble zinc ore and precipitate impurities such as iron and silica to make filtrate filterable, none of the above-mentioned techniques addresses a process to recover zinc from zinc bearing solutions. Most of the above-mentioned processes use conventional electrowinning process to recover zinc from zinc sulphate solutions. Electrowinning is well known technology, however, it requires high capital cost. Therefore, it is only suitable technology if the metal value is significant enough to build an electrowinning plant or, otherwise, a recovery process has to be located at and integrated with present zinc refineries.
Another problem with the conventional electrowinning process is that the zinc concentration level must be higher than a certain level to operate. Therefore, if the zinc concentration in the final filtrate of the leaching operation is not high enough, the zinc concentration in the filtrate of leached acid soluble zinc materials has to be upgraded.
Zinc in the solution can be concentrated by several known methods, such as solvent extraction or ion exchange. However, those processes have several drawbacks, such as limitation of extraction capacity, organic substance contamination and high capital cost. In addition, these approaches still require a conventional electrowinning process to recover zinc from zinc bearing solution.
Another approach to recover zinc from the zinc bearing solution is to precipitate zinc from the solution itself The precipitated zinc now can be easily sent to the zinc refinery or other facilities which requires such materials. Zinc can be precipitated from zinc sulphate solution as zinc oxide, zinc hydroxide, or basic zinc sulphate by adding alkaline. Also, zinc can be precipitated as zinc sulphide by adding sulphide or as zinc carbonate by adding carbon dioxide. However, the operation of the process for producing zinc sulphide or zinc carbonate is expensive. Also, producing zinc hydroxide causes severe filtering problems and producing basic zinc sulphate results in high transportation cost since the grade of zinc in basic zinc sulphate is very low. Producing zinc oxide is the most suitable approach, but unless you are using sodium hydroxide, which is very expensive reagent, the zinc oxide will be contaminated with a large amount of gypsum that is co-precipitated during zinc oxide precipitation.
Therefore, it is desirable to develop a process for producing zinc oxide from zinc sulphate solution that is originated from acid soluble zinc materials.
It is an object of the invention to provide an apparatus for producing zinc oxide from an acid soluble zinc-containing material. The apparatus comprises a leaching unit for leaching the acid soluble zinc-containing material with sulphuric acid, a first precipitating unit for precipitating iron, silica and other metals (Al, As, Sb, etc.) from the leach solution using calcium oxide (or Ca(OH)2, MgO, ZnO), and removing, copper, cadmium, nickel and cobalt from the leach solution by cementation with zinc dust. A second precipitating unit is used to precipitate zinc oxide from the leach solution using calcium oxide (or Ca(OH)2). Then the precipitated zinc oxide is separated from calcium sulphate by flotation with cationic collectors such as dodecylamine hydrochloride, and dodecylamine sulphate, and/or potassium amyl xanthate as a collector. This separation of zinc oxide from calcium sulphate also can be achieved by granulometric sizing. In addition, a combination of the two techniques (sizing and flotation) can be used for separating of zinc oxide from calcium sulphate. In addition, the tailing of these steps can be recycled to the leaching step to recover zinc value.
A further object of the present invention is to provide a process for producing zinc oxide from an acid soluble zinc-containing material. The process comprises the steps of leaching the acid soluble zinc material with sulphuric acid, precipitating iron, silica and other metals (Al, As, Sb, etc.) from the leach solution using calcium oxide (or Ca(OH)2, MgO, ZnO), cementing copper, cadmium, nickel and cobalt in the leach solution with zinc dust, and precipitating zinc oxide from the leach solution using calcium oxide (or Ca(OH)2). Then the precipitated zinc oxide is separated from calcium sulphate by flotation with cationic collectors such as dodecylamine hydrochloride, and dodecylamine sulphate, and/or potassium amyl xanthate. The separation of zinc oxide from calcium sulphate can also be achieved by granulometric sizing. In addition, a combination of the two techniques (granulometric sizing and flotation) can be used for separating zinc oxide from calcium sulphate. The tailing of these steps can also be recycled to the leaching step to recover zinc value.
Still another object of the present invention is to provide a zinc oxide precipitate that is substantially free of basic zinc sulphate by leaching an acid soluble zinc-containing material with sulphuric acid. Iron, silica and other metals (Al, As, Sb, etc.) are precipitated from the leach solution using calcium oxide (or Ca(OH)2, MgO, ZnO), and copper, cadmium, nickel and cobalt are cemented from the leach solution using zinc dust. Zinc oxide is precipitated from the leach solution using calcium oxide.
A further object of the present invention is to provide a process for recovering copper and cadmium from an acid soluble zinc-containing material. The process comprises the steps of leaching the acid soluble zinc material with sulphuric acid, precipitating iron, silica and other metals (Al, As, Sb, etc.) from the leach solution using calcium oxide (or Ca(OH)2, MgO, ZnO) and recovering copper and cadmium from the leach solution by cementing with zinc dust.
Still another object of the present invention is to provide a process for separating zinc oxide from gypsum, which precipitates during zinc oxide precipitation. The process comprised the steps of leaching the acid soluble zinc-containing material with sulphuric acid, precipitating iron, silica and other metals (AL, As, Sb, etc.) from the leach solution using calcium oxide (or Ca(OH)2, MgO, ZnO), and cementing copper, cadmium, nickel and cobalt from the leach solution using zinc dust. The precipitated zinc oxide is separated from co-precipitated gypsum by flotation with cationic collectors such as dodecylamine hydrochloride, and dodecylamine sulphate, and/or potassium amyl xanthate as a collector. The separation of zinc oxide from calcium sulphate can also be achieved by granulometric sizing. In addition, a combination of the two techniques (granulometric sizing and flotation) can be used for separating zinc oxide from calcium sulphate. The tailing of the flotation step can also be recycled to the leaching step to recover zinc value.
The invention is described in more detail below with reference to the accompanying figure.